It is known that certain materials, which exist in a stable condition at room temperature, in either a substantially amorphous or substantially crystalline state can be converted from one of these states to the other by supplying sufficient energy to heat the material and then allowing it to cool under controlled conditions to a crystalline or an amorphous state.
It is also known that such materials may be used for storing and retrieving information. These materials may be used in the form of thin films for optical recordings. The thin films are written upon by selectively changing particular areas of the thin film from one state to the other (from amorphous to crystalline or crystalline to amorphous). Such change may be accomplished by providing a low amplitude energy pulse of sufficient duration to heat the material above its transition temperature. On the other hand to convert the material from a crystalline to an amorphous state, rapid cooling is essential. This change, may be accomplished by pulsing the material with a high energy pulse source to raise the material to the melt temperature after which there must be a rapid drop in temperature freezing the material in the amorphous state before crystallization can occur.
Optical recording elements comprising thin films of GeTe alloys are known for use as optical recording layers. One such optical element is disclosed in papers entitled "New Ideas for Phase-Change Media Reversible Media--Achieving Sub-Microsecond Erase with Data Stability", Chen et al and "Systematic Phase Transformation Kinetics Measurements--Crystallization and Critical Quench Rates of the Binary Te-Ge System", Rubin et al. Both of the latter papers were presented at the Topical Meeting on Optical Data Storage, IEEE, and OSA in Washington, D.C. on Oct. 15-17, 1985.
In these papers, Ge-Te thin films are used for erasable recordings. In erasable recording the films have to be crystallized first using a long duration laser beam to form the "erased" condition. Data is then written on the film as localized amorphous spots using a high intensity short duration focused laser beam to raise the film above its liquidus temperature. The film is then quenched thereby forming the amorphous or written condition. Subsequent erasure of the data is done again by laser crystallization which is usually a slow process resulting in excessively long erasure time for many applications. The essence of these reports is that by using a stoichiometric GeTe composition, the crystallization rate can be significantly improved to allow laser pulse lengths as low as 250 nanoseconds for crystallization.
The problem is that such films cannot be used in write-once optical recording elements in which the written information is encoded in the crystallized state. The slow crystallization rate from the initial amorphous state makes the writing process prohibitively slow for most applications.
In U.S. Pat. No. 4,710,452, there is described an optical recording element comprising a thin film of a Ge-Te-Ti alloy. The incorporation of Ti yielded improved performance over the above-described Ge-Te thin films in terms of a higher rate of crystallization from the amorphous state, high recording sensitivity and high carrier-to-noise ratios (CNR). An improved recording medium containing Ge-Te-Sb is also disclosed in U.S. Pat. No. 4,670,345 as having excellent thermal stability, sensitivity and signal-to-noise ratio.
The problem is that although the above patented ternary compounds exhibit improved performance, the prior art gives no guidance that would lead to the selection of particular metals which would be operable for this purpose. Further, there appears to be no general rule which a skilled worker in the art may follow to arrive at the operable metals. For example, it has been demonstrated that optical recording elements made of Ge-Te-Cr alloy exhibit no significant improvement over the Ge-Te alloy even though Cr is near Ti on the periodic table of elements. Also, even after the skilled worker discovers which metals are useful for the invention, it is further necessary to ascertain the composition which gives the desired improvement.
Thus, the problem to be solved is to provide alternative ternary alloys having performance similar to Ge-Te-Ti.